Adjustable, self-locking load supporting mechanism for booms

ABSTRACT

An infinitely adjustable self-locking stirrup for mounting intermediate load hooks on an extensible boom attachment for fork lift trucks is disclosed. The mounting includes a U-shaped yoke supported on the top of one of the horizontal extensible boom sections and a pivoted clevis. The clevis is supported in the legs of the yoke closely adjacent the underside of the boom section and provides a mounting for the load hook. When the vertical axis of the clevis is maintained normal to the bottom of the boom section, a top surface on the clevis is parallel to, and has a slight clearance with, the bottom of the boom section so the stirrup mounting is free to slide along the boom section. However, when the clevis is tilted slightly, the top surface wedges against the bottom of the boom section and locks the stirrup mounting in a given position.

United States Patent [1 1 Glass et al.

[451 Sept. 18,1973

[ ADJUSTABLE, SELF-LOCKING LOAD Primary Examiner-Richard E. AegerterSUPPORTING MECHANISM FOR BOOMS Assistant ExaminerMerle F. Maffei [75]Inventors: Clifford A. Glass, Chicago; Robert E. Atmmey F Anderson atKraft, Naperville; Edward W. Schmidt, Chicago, all of I11. [57] ABSTRACT[73] Assignee: FMC Corporation, San Jose, Calif. An infinitelyadjustable self-locking stirrup for mounting intermediate load hooks onan extensible boom at- [22] 1972 tachment for fork lift trucks isdisclosed. The mounting [21] Appl. No.: 217,900 includes a Ushaped yokesupported on the top of one of the horizontal extensible boom sectionsand a pivoted clevis. The clevis is supported in the legs of the 212/55212/ kg? g yoke closely adjacent the underside of the boom sec- [58] Fi394/82 tion and provides a mounting for the load hook. When e 0 a c 3the vertical axis of the clevis is maintained normal to 1 the bottom ofthe boom section, a top surface on the clevis is parallel to, and has aslight clearance with, the [56], cued bottom of the boom section so thestirrup mounting is UNITED STATES PATENTS free to slide along the boomsection. However, when 2,867,333 1/1959 De Shano 212/55 X the clevis istilted slightly, the top I surface wedges 248.949 11/1331 24/ PJ againstthe bottom of the boom section and locks the 21487997 11/1949 24/263 Mstirrup mounting in a given position. 3,116,840 1/1964 Carbert et al212/55 X 9 Claims, 7 Drawing Figures l4 V I Z2 8 C i it" my 26 8 n \20@I a T'IE I PAIENTED 3.759.399

saw 2 0F 3 FIB EI ADJUSTABLE, SELF-LOCKING LOAD SUPPORTING MECHANISM FORBOOMS BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention relates to load supporting mechanism for booms, and, moreparticularly to an extensible boom attachment with lifting hooksintermediate the ends of the extensible boom for a fork lift truck orother front end load vehicles. The attachment permits elongated articlesto be picked up and carried by the vehicle.

2. Description of the Prior Art Telescopic booms for hoisting cranes andthe like are old as shown by US Pat Nos. 734,973 to J. M. Shively,2,147,313 to R. Payson, 2,417,031 to A. E. witherell, 3,059,785 to K. S.Buckeye, 3,165,157 to R. E. Carbert, and 3,306,467 to S. O. Weempe.Various types of detachable suspension or gripping devices for beams areshown in US. Pat. Nos. 1,894,895 to Steele et al.,

2,444,055 to D. B. McAulay, 2,961,269 and 2,961,270 r SUMMARY. OF THEINVENTION The apparatus of the present invention comprises a freelyadjustable, self-locking mounting arrangement for a load hook on a boom.Preferably, the hook, or hooks, are mounted on a telescopic orextensible boom attachment for use with a fork lift truck or similarvehicle. In the preferred embodiment of the invention, the mountingarrangement comprises a U-shaped stirrup which slides over the top of,and is supported by, a givensection of a rectangular or squareboorn. Aclevis is pivotally mounted transversely of the boom in the lower end ofthe stirrup closely adjacent to the underside of the boom section and aswivel load hook is pivotally attached to the lower portion of theclevis. When the mounting stirrup with the load, hook perpendicular tothe longitudinal axis of the boom is pushed or pulled parallel to thislongitudinal axis of the boom, the stirrup is free to slide easily alongthe boom toan infinite number of positions. However, when the load hookis moved to an angular position along the longitudinal axis of the boomslightly less than 90 'with the bottom of the boom, a top surfaceof theclevis wedges against the bottom of the boom section and locks the hookat this position. This might occur when a load is. suspendedsimultaneously from two spaced hooks with non-parallel slings. The samelocking action can also occur if a load is suspended vertically from theintermediate hook and the boom is tilted longitudinally, as when thevehicle is negotiating an incline or decline or the mast of the truck istilted backwardly or forwardly, so the relative position of the boom andthe clevis for the load hook is no longer at right angles.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of theintennediate load hook of the invention shown on each section of an ex-2 tensible boom attachment mounted on a fork lift truck.

FIG. 2 is an enlarged partial elevational view of the extended boom ofFIG. 1 showing an intermediate load hook assembly of the invention oneach section of the extensible boom.

FIG. 3 is an enlarged partial elevational view of the extended boom,similar to FIG. 3, but showing a different boom attachment in which theinner boom of FIG. 2 is the same size as the outer boom of FIG. 3 toillustrate that the same stirrup for an intermediate load hook assemblymay be used interchangeably on different boom sections of differentattachments.

FIG. 4 is a cross-sectional view of the extended inner boom section withthe intermediate load hook mounted thereon, the view being taken alongline 4-4 on FIG. 2.

FIG. 5 is a cross-sectional view of the boom and the intermediate loadhook, showing the locked position of the clevis with the boom tilted.

FIG. 6 is a transverse elevational view of the clevis for theintermediate book of FIG. 3.

FIG. 7 is an end view of the clevis of FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENT An extensible boom attachment,generally designated 10, mounted .on forks 12 of a fork lift truck 14 isshown in FIG. 1. Chain 15 locks the attachment to the elevating carriageof the truck 14. The attachment comprises a frame 16 having openings 17for receiving the forks 12 of the lift truck, a fixed outer boom section18, a sliding inner boom section 19, and intermediate load supportingmechanism or devices 20, 20'- The outer boom section 18 is fixed to andsupported in the upper portion of frame 16 with a substantial portion ofboom section 18 extending normally horizontally beyond frame 16. Theinner boom section 19 is matingly and slidingly supported within theouter boom section 18 permitting telescoping of the inner boom section19 into the outer boom section 18. The sliding inner boom section 19 maybe manually extended to any desired position witin its working length.Intermediate load supporting mechanisms 20, 20' are shown on the fixedouterand sliding inner boom sections respectively to provide infinitelyadjustably positioned points for attaching hooks and slings'to anelongated article to be inner boom section of an intermediate model maybeused as the fixed outer: boom section of the next smaller model in theseries and the fixed outer boom section of the intermediate model may beused as the sliding inner boom section of the next larger model in theseries. Thus economy of manufacture, reduction in the total number ofparts in the various models and interchangeability of some parts isobtained such as the adjustable self-locking load supporting devices.

The loadsupporting device 20, 20' or 20" comprises a stirrup or slidemember 22, an upper pin 24, a locking member or clevis 25, a lower pin26, a swivel load hook 27 and locking pins 28, 28. The stirrup or slidemem-' project downwardly adjacent the vertical sides of the boom sectionand below the bottom of the boom section. A hole-29 is provided in thelower end of each leg of the yoke member for pin 24 to pass through bothlegs and to support the locking member 25 transversely of the boomsection. Locking pins 28, such as cotter pins, hold pin 24 at each ofits ends in stirrup member 22. Pin 24 is removable from yoke member 22.

The locking member or clevis 25 comprises two spaced apart transversemembers 30 and two spaced apart longitudinal members 32 secured to eachof the transverse members 30 at right angles just slightly below the topedge locking surface 34 of the upper portion of members 30 as shown inN68. 6-7. An upper hole 35 and a lower hole 36, in vertical alignmentwith hole 35, are provided in members 32 so that pins 24 and 26respectively can pass therethrough transversely of the boom section.Holes 35 have a slight clearance over pin 24 so that the clevis 25 canpivot freely on pin 24 when assembled between the legs of stirrup member22. Holes 35 in members 32 of the clevis are located with respect to topsurface 34 of member 30, as indicated by dimension A (FIG. 7), so thatwhen the clevis 25 is assembled in stirrup 22 on pin 24 there will beonly a slight clearance B (FIG. 4) between the bottom surface of theboom section and the top surface 34 of member 30 when those surfaces areparallel. This clearance is normally in the order of one-sixteenth of aninch. The significance of this clearance dimension B will be brought outlater in this description. Lower pin 26 in the lower hole 36 of members32 pivotally supports load hook 27. Load hook 27 has a shackle 38 whichloosely fits over pin 26 to provide pivotal movement for the load hook27 in a direction parallel to the longitudinal axis of the boom section.Load hook 27 further has a swivel connection between the shackle and thehook itself so that the hook can rotate about a vertical axis in shackle38. After hook 27 is assembled on pin 26 in clevis 25, looking pins 28,such as cotter pins, are provided at each end of pin 26 to preventendwise movement of pin 26 in clevis 25. It is to be noted that bothpins, 24 and 26, are horizontal and are positioned transversely of theaxis of the boom sections so the pivotal movement of clevis 25 in thestirrup 22 and of shackle 38 in the lower endof clevis 25 islongitudinal of and parallel to the axis of the boom section on which itis mounted. The swivel mounting of hook 27 in its shackle 38 gives thehook complete rotational movement about a vertical axis.

The clearance B (FIG. 4) between the top edge locking surface 34 of thetransverse members of the clevis member 25 and the bottom surface of theboom section when those surfaces are parallel permits the loadsupporting devices to slide freely along the boom section to anyintermediate position within its length when pushed or pulled parallelto the longitudinal axis of the boom thus giving the hook an infinitenumber of positions. However, if the hook, when supporting a load, ismoved to an angle slightly less than 90 with the longitudinal axis ofthe boom section, the clevis will pivot on pin 24 and this will causethe top surface 34 of one of the clevis members 30 to wedge against andbite into the bottom surface of the boom section since the clearance Bwill be reduced to zero at one of the members 30. Similarly, when theboom tilts slightly as when the truck is negotiating an incline ordecline and a load on the hook holds the hook and the clevis in avertical position, one top surface of the clevis will bite into thebottom surface of the boom, as shown in FIG. 5, and lock the clevis andhook in a fixed longitudinal position on the boom. This condition alsooccurs when the mast on a fork lift truck is tilted backwardly orforwardly slightly, thus inclining the extensible boom even though thetruck is traversing on a horizontal plane. This is a frequently usedoperating procedure. With this device it is impossible for the load tobecome unintentionally unclamped at any inclination of the boom from thehorizontal. The self-locking hook assembly also prevents movement of thestirrup when the truck makes quick starts and stops which might causethe load on the hooks to swing or when a load is suspendedsimultaneously from two spaced hooks with nonparallel (i.e., notvertical) slings.

Thus an unloaded hook on either or both boom sections may be easilypositioned to any desired location for the article to be lifted andcarried by the truck. However, the hook will be locked to the boom wheneither the boom or hook is tilted for any reason, enabling the hook toresist any longitudinal forces at the hook tending'to change theposition of the hook. It is also to be noted that even while the stirrupis supporting a' moderate load, the adjustment of the stirrup can stillbe made manually so long as the top surface of the clevis is parallel tothe underside of the boom section on which it is installed. Further thisstirrup mounting permits easy removal of a hook assembly by simplyremoving the clevis 25 and its pin 24 permitting the-yoke or stirrupmember to be liftedoff over the top of the boom section. Likewise, anadditional load hook may be easily'added at any time without having todisassemble the boom sections. An adjustable, self-locking, easilyremovable stirrup mounting for a hook assembly on a boom is thereforeprovided.

The longitudinal direction as referred to in this description is thatdirection parallel to the long axis of the extensible boom sections. Thetransverse direction is perpendicular to the long axis of the extensibleboom sections. 1

Although the best mode contemplated for carrying out the presentinvention has been shown and described, it will be'apparent thatmodification and variation maybe made without departing from what isregarded to be the'subject matter of the invention as defined by theattached claims.

What is claimed is:

l. A load supporting device for mounting on a boom comprising a slidemember movable along the boom when not locked thereto, said slide memberpartially encircling the boom, said slide member having legs dependingbelow the boom, and a locking member pivotally mounted on the legs ofsaid slide member in close proximity to the bottom of the boom, saidlocking member having means to receive a load, said locking membershiftable into locking engagement with the boom in response to a shiftfrom a perpendicular relationship between the longitudinal direction ofthe boom and the direction of the force of the load.

2. The load supporting device of claim 1, wherein said locking membercomprises an upper portion and a lower portion, said upper portionincluding a top locking surface closely adjacent and parallel to thebottom of the boom, said lower portion including th means to support aload.

3. The load supporting device of claim 2, wherein said locking memberfurther comprises a pair of vertical longitudinally positioned platemembers, each having an upper hole and a lower hole therethrough invertical alignment, the horizontal axes of said holes being parallel toeach other and to the bottom of said boom, the axes of said holes alsobeing transverse to the longitudinal axis of the boom; at least onetransverse bar member to which said longitudinally positioned platemembers are securely attached in spaced relationship, said transversebar having a flat horizontal top locking surface projecting above saidlongitudinally positioned plate members and located adjacent the bottomof the boom; and a pin removably supported in the lower ends of the legsof said slide member in a direction transverse to the longitudinal axisof the boom, said pin passing through said upper holes providing pivotalmovement of said locking member, said lower holes providing for thesupport of a load. a

4. A load supporting mechanism comprising in combination: a boom, aslide member mounted on the boom for movement therealong when not lockedthereto, and a locking member comprising a clevis member and a pin, saidclevis member having a flat horizontal top locking surface normallypositioned parallel to the underside of said boom, said pin removablysupported in the lower end of said slide member, said clevis membermounted on said pin to pivot in a plane parallel to the longitudinalaxis of said boom with said top locking surface closely adjacent thebottom of said boom, said locking member normally held in apredetermined relationship to said boom by the force of the load, saidtop locking surface of said locking member spaced from said boom whilesaid predetermined relationship is maintained, said top locking surfaceengaging said boom to lock said slide member to said boom when saidpredetermined relationship is altered, said slide member being free toslide along the. longitudinal axis of saidboom when said top lockingsurface is parallel to the underside of said boom, said slide memberbeing locked in a'fixed position relative to said boom when said clevismember is tilted slightly on said pin causing wedging of said toplocking surface against the bottom of said boom.

5. The load supporting mechanism of claim 4, wherein said clevis membercomprises a pair of longitudinal vertically positioned plate members, apair of transverse vertically positioned spaced apart plate members, anda pin removably mounted in the lower endof said slide member, eachlongitudinal plate member having an upper hole and a lower holetherethrough with the axes of said holes being parallel to each otherand to the bottom of the boom transverse to the longitudinal axis of theboom, said transverse plate members being perpendicularly secured tosaid longitudinal plate members in spaced apart relationshipsymmetrically about a vertical centerline through the upper and lowerholes, said transverse plate members each having a flat, horizontal topedge locking surface projecting above said longitudinal plate membersand positioned closely adjacent the underside of the boom, .said pinpassing through said upper holes of said longitudinal plate membersproviding pivotal movement of said clevis member in a direction alongthe longitudinal axis of the boom, said lower holes of said longitudinalplate members providing for the support of a load, whereby when saidclevis member is tilted slightly in one direction, the

top edge surface of one of said transverse plates will wedge against theunderside of the boom and when said clevis member is tilted slightly inthe opposite direction, the top edge surface of the other of saidtransverse plate members will wedge against the underside of the boomproviding locking of said slide member in either direction of movement.

6. In an extensible boom attachment for a lift truck having an elevatingcarriage, the attachment having a frame removably supported ontheelevating carriage and having an extensible boom comprising an outerboom section supported in and projecting substantially horizontally fromthe frame and an inner boom section being slidingly and telescopinglyreceived in the outer boom section, the improvement of an adjustableselflocking load support mechanism which comprises:

'a. a yoke member slidingly supported on either one of the boomsections, said yoke member mounted on the one boom section for movementtherealong in an infinite number of positions between the ends of theone boom section when not locked thereto, said yoke member partiallyencircling the one boom section, said yoke member having a pair of legsextending downwardly below the bottom of the one boom section and havinga pin removably mounted ,in the lower ends of said legs; and

b. a locking member pivotally mounted on said pin of said yoke member,said locking member having means to support a load and having a lockingportion closely adjacent and parallel to the bottom of the one boomsection, said locking memberv normally held in a predeterminedrelationship to the one boom section by the force of the load andsaidlocking portion spaced from the one boom section while saidpredetermined relationship is main tained, said locking portion engagingthe one boom section to lock said yoke member to the one boom i sectionwhensaid predetermined relationship is altered by tilting of the oneboom section from a horizontal position or by tilting of said lockingmember from a vertical direction by the force of the load supported onsaid locking member, said yoke member being easily removed from the oneboom sec-, tion by removing said pin and said locking member.

7. i The extensible boom attachment of claim 6 wherein said lockingmember has at least one upper hole to receive said pin and has at leastone lower hole vertically, below the upper hole to support a load, saidlocking portion of said locking member being perpendicular to acenterline through the upper and lower holes, said yoke member beingfree to slide along said one boom section when the center-line throughthe upper and lower holes remains perpendicular to the bottom of saidone boom section, but said yoke member is locked to said one boomsection whenthe perpendicular relation of the centerline through theupper and lower holes in said locking member to the bottom of said oneboom section is changed, said locking portion no longer being parallelto the bottom of said one boom section and wedging of said lockingportion against the bottom of said one boom section results.

8. An adjustable, self-locking load supporting mechanism for mounting onaboom comprising: a boom; a slide member mounted on said boom andmovable therealong when not locked thereto, said slide member having atleast one leg depending below said boom; and a locking member pivotallymounted on the lower end of said leg, said locking member having meansto receive a load and having a locking portion spaced from the bottom ofsaid boom in a predetermined clearance relationship when said boom ishorizontal and when the force of the load is acting vertically, saidlocking portion engaging the bottom of said boom to lock said slidemember to said boom when said predetermined relationship is altered byinclining said boom from a horizontal position or by tilting saidlocking member due to a change in direction of the force of the loadaway from the vertical.

9. In an extensible boom attachment for a lift truck having an elevatingcarriage, the attachment having a frame removably supported on theelevating carriage and having an extensible boom supported by the frameand projecting substantially horizontally from the frame, the extensibleboom comprising a first boom section slidingly and telescopinglyreceived in a second boom section, the improvement of an adjustableselflocking load support mechanism which comprises:

a. a yoke member slidingly supported on either one of the boom sectionsand movable therealong in an infinite number of positions between theends of the one boom section when not locked thereto, said yoke memberhaving at least one leg depending below the one boom section;

b. a locking member pivotally mounted on the lower end of said one legof said yoke member, said locking member having means to support a loadand having a locking portion spaced from the bottom of the one boomsection in a predetermined clearance relationship when the boom sectionis horizontal and when the force of the load is acting vertically, saidlocking portion engaging the bottom of the one boom section to lock saidyoke member to the one boom section when said predetermined relationshipis altered by inclining the one boom section or by tilting said lockingmember due to a change in direction of the force of the load away fromthe vertical. v

1. A load supporting device for mounting on a boom comprising a slidemember movable along the boom when not locked thereto, said slide memberpartially encircling the boom, said slide member having legs dependingbelow the boom, and a locking member pivotally mounted on the legs ofsaid slide member in close proximity to the bottom of the boom, saidlocking member having means to receive a load, said locking membershiftable into locking engagement with the boom in response to a shiftfrom a perpendicular relationship between the longitudinal direction ofthe boom and the direction of the force of the load.
 2. The loadsupporting device of claim 1, wherein said locking member comprises anupper portion and a lower portion, said upper portion including a toplocking surface closely adjacent and parallel to the bottom of the boom,said lower portion including the means to support a load.
 3. The loadsupporting device of claim 2, wherein said locking member furthercomprises a pair of vertical longitudinally positioned plate members,each having an upper hole and a lower hole therethrough in verticalalignment, the horizontal axes of said holes being parallel to eachother and to the bottom of said boom, the axes of said holes also beingtransverse to the longitudinal axis of the boom; at least one transversebar member to which said longitudinally positioned plate members aresecurely attached in spaced relationship, said transverse bar having aflat horizontal top locking surface projecting above said longitudinallypositioned plate members and located adjacent the bottom of the boom;and a pin removabLy supported in the lower ends of the legs of saidslide member in a direction transverse to the longitudinal axis of theboom, said pin passing through said upper holes providing pivotalmovement of said locking member, said lower holes providing for thesupport of a load.
 4. A load supporting mechanism comprising incombination: a boom, a slide member mounted on the boom for movementtherealong when not locked thereto, and a locking member comprising aclevis member and a pin, said clevis member having a flat horizontal toplocking surface normally positioned parallel to the underside of saidboom, said pin removably supported in the lower end of said slidemember, said clevis member mounted on said pin to pivot in a planeparallel to the longitudinal axis of said boom with said top lockingsurface closely adjacent the bottom of said boom, said locking membernormally held in a predetermined relationship to said boom by the forceof the load, said top locking surface of said locking member spaced fromsaid boom while said predetermined relationship is maintained, said toplocking surface engaging said boom to lock said slide member to saidboom when said predetermined relationship is altered, said slide memberbeing free to slide along the longitudinal axis of said boom when saidtop locking surface is parallel to the underside of said boom, saidslide member being locked in a fixed position relative to said boom whensaid clevis member is tilted slightly on said pin causing wedging ofsaid top locking surface against the bottom of said boom.
 5. The loadsupporting mechanism of claim 4, wherein said clevis member comprises apair of longitudinal vertically positioned plate members, a pair oftransverse vertically positioned spaced apart plate members, and a pinremovably mounted in the lower end of said slide member, eachlongitudinal plate member having an upper hole and a lower holetherethrough with the axes of said holes being parallel to each otherand to the bottom of the boom transverse to the longitudinal axis of theboom, said transverse plate members being perpendicularly secured tosaid longitudinal plate members in spaced apart relationshipsymmetrically about a vertical centerline through the upper and lowerholes, said transverse plate members each having a flat, horizontal topedge locking surface projecting above said longitudinal plate membersand positioned closely adjacent the underside of the boom, said pinpassing through said upper holes of said longitudinal plate membersproviding pivotal movement of said clevis member in a direction alongthe longitudinal axis of the boom, said lower holes of said longitudinalplate members providing for the support of a load, whereby when saidclevis member is tilted slightly in one direction, the top edge surfaceof one of said transverse plates will wedge against the underside of theboom and when said clevis member is tilted slightly in the oppositedirection, the top edge surface of the other of said transverse platemembers will wedge against the underside of the boom providing lockingof said slide member in either direction of movement.
 6. In anextensible boom attachment for a lift truck having an elevatingcarriage, the attachment having a frame removably supported on theelevating carriage and having an extensible boom comprising an outerboom section supported in and projecting substantially horizontally fromthe frame and an inner boom section being slidingly and telescopinglyreceived in the outer boom section, the improvement of an adjustableself-locking load support mechanism which comprises: a. a yoke memberslidingly supported on either one of the boom sections, said yoke membermounted on the one boom section for movement therealong in an infinitenumber of positions between the ends of the one boom section when notlocked thereto, said yoke member partially encircling the one boomsection, said yoke member having a pair of legs extending downwardlybelow the bottom of the one boom Section and having a pin removablymounted in the lower ends of said legs; and b. a locking memberpivotally mounted on said pin of said yoke member, said locking memberhaving means to support a load and having a locking portion closelyadjacent and parallel to the bottom of the one boom section, saidlocking member normally held in a predetermined relationship to the oneboom section by the force of the load and said locking portion spacedfrom the one boom section while said predetermined relationship ismaintained, said locking portion engaging the one boom section to locksaid yoke member to the one boom section when said predeterminedrelationship is altered by tilting of the one boom section from ahorizontal position or by tilting of said locking member from a verticaldirection by the force of the load supported on said locking member,said yoke member being easily removed from the one boom section byremoving said pin and said locking member.
 7. The extensible boomattachment of claim 6 wherein said locking member has at least one upperhole to receive said pin and has at least one lower hole verticallybelow the upper hole to support a load, said locking portion of saidlocking member being perpendicular to a centerline through the upper andlower holes, said yoke member being free to slide along said one boomsection when the centerline through the upper and lower holes remainsperpendicular to the bottom of said one boom section, but said yokemember is locked to said one boom section when the perpendicularrelation of the centerline through the upper and lower holes in saidlocking member to the bottom of said one boom section is changed, saidlocking portion no longer being parallel to the bottom of said one boomsection and wedging of said locking portion against the bottom of saidone boom section results.
 8. An adjustable, self-locking load supportingmechanism for mounting on a boom comprising: a boom; a slide membermounted on said boom and movable therealong when not locked thereto,said slide member having at least one leg depending below said boom; anda locking member pivotally mounted on the lower end of said leg, saidlocking member having means to receive a load and having a lockingportion spaced from the bottom of said boom in a predetermined clearancerelationship when said boom is horizontal and when the force of the loadis acting vertically, said locking portion engaging the bottom of saidboom to lock said slide member to said boom when said predeterminedrelationship is altered by inclining said boom from a horizontalposition or by tilting said locking member due to a change in directionof the force of the load away from the vertical.
 9. In an extensibleboom attachment for a lift truck having an elevating carriage, theattachment having a frame removably supported on the elevating carriageand having an extensible boom supported by the frame and projectingsubstantially horizontally from the frame, the extensible boomcomprising a first boom section slidingly and telescopingly received ina second boom section, the improvement of an adjustable self-lockingload support mechanism which comprises: a. a yoke member slidinglysupported on either one of the boom sections and movable therealong inan infinite number of positions between the ends of the one boom sectionwhen not locked thereto, said yoke member having at least one legdepending below the one boom section; b. a locking member pivotallymounted on the lower end of said one leg of said yoke member, saidlocking member having means to support a load and having a lockingportion spaced from the bottom of the one boom section in apredetermined clearance relationship when the boom section is horizontaland when the force of the load is acting vertically, said lockingportion engaging the bottom of the one boom section to lock said yokemember to the one boom section when said predetermined relationship isaltered by inclining the one boom section or by tilting said lockingmember due to a change in direction of the force of the load away fromthe vertical.